1. Field of the Invention
The present invention relates to an optical sensor having a current amplifier, and a display apparatus provided with the optical sensor.
2. Description of the Related Art
A non-emissive display apparatus, such as a liquid crystal display apparatus, displays an image by lighting a display unit on which pixel matrixes are arranged. A display unit can be lit by two methods. One uses external light around a display apparatus, and the other uses a light source (backlight) provided at the rear of a display unit. Generally, the former is called a reflective type display apparatus, and the latter is called a transmissive type display apparatus.
It is known that the viewability in a transmissive type display apparatus varies with the brightness around a display apparatus. For example, the viewability is good when the surrounding of a display apparatus is dark, and vice versa.
Jpn. Pat. Appln. KOKAI Publication No. 2000-112382 discloses a method of improving the viewability in a display apparatus by detecting the illuminance of light (external light) around a display apparatus by an optical sensor, and controlling the illuminance of backlight according to the illuminance of detected external light.
As a method of providing a compact sensor having a large dynamic range as an optical sensor for detecting the illuminance of light, Jpn. Pat. Appln. KOKAI Publication No. 2003-75252 discloses a technique to amplify a photocurrent signal detected by a photodiode constituting an optical sensor, by using a bipolar transistor.
When an optical sensor is used to detect the brightness around the display apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2000-112382, if an amorphous thin-film transistor (hereinafter, called an a-Si TFT) is used for the optical sensor, the optical sensor and display unit can be manufactured in the same process. Here, the optical sensor using an a-Si TFT is assumed to detect a current (called a leak current) flowing between a drain and a source of a TFT according to the illuminance of light, when a negative voltage (e.g., −5V to −10V) is applied to a gate electrode of the TFT.
Generally, a leak current of a TFT is very small, and it is desirable to take out the leak current from an optical sensor after amplifying, to use an a-Si TFT optical sensor for controlling the luminance of backlight disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2000-112382. The technique disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2003-75252 amplifies an output current from an optical sensor by using a bipolar transistor. Therefore, when the technique disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2003-75252 is used, as a TFT and a bipolar transistor are manufactured in different processes, a display unit and an optical sensor cannot be manufactured in the same process, and the number of manufacturing steps is inevitably increased.